The present disclosure relates to a coordinate measuring device. One set of coordinate measurement devices belongs to a class of instruments that measure the three-dimensional (3D) coordinates of a target point by sending a beam of light to the point. The beam of light may impinge directly on the point or on a retroreflector target in contact with the point. In either case, the instrument determines the coordinates of the target point by measuring a distance and two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. The beam may be steered with a gimbaled mechanism, a galvanometer mechanism, or other mechanism.
A tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more beams it emits, which may include light from a laser or non-laser light source. Coordinate-measuring devices closely related to the tracker include a total station. A total station is a 3D measuring device most often used in surveying applications. It may be used to measure the coordinates of a diffusely scattering target or a retroreflective target. Hereinafter, the term tracker is used in a broad sense to include trackers as well as total stations and to include dimensional measuring devices that emit laser or non-laser light.
In many cases, a tracker sends a beam of light to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The vertex, which is the common point of intersection of the three mirrors, is located at the center of the sphere. Because of this placement of the cube corner within the sphere, the perpendicular distance from the vertex to any surface of the SMR rests remains constant, even as the SMR is rotated. Consequently, the tracker can measure the 3D coordinates of a surface by following the position of an SMR as it is moved over the surface. Stating this another way, the tracker measures only three degrees of freedom (one radial distance and two angles) to fully characterize the 3D coordinates of a surface.
One type of tracker contains only an interferometer (IFM) without an absolute distance meter (ADM). If an object blocks the path of the beam of light from one of these trackers, the IFM loses its distance reference. The operator must then track the retroreflector to a known location to reset to a reference distance before continuing the measurement. A way around this limitation is to put an ADM in the tracker. The ADM can measure distance in a point-and-shoot manner. Some trackers contain only an ADM without an interferometer.
A gimbal mechanism within the tracker may be used to direct a beam of light from the tracker to the SMR. Part of the light retroreflected by the SMR enters the tracker and passes onto a position detector. A control system within the tracker uses position of the light on the position detector to adjust the rotation angles of the mechanical axes of the tracker to keep the beam of light centered on the SMR. In this way, the tracker is able to follow (track) a moving SMR.
Angle measuring devices such as angular encoders are attached to the mechanical axes of the tracker. The one distance measurement and two angle measurements of the tracker are sufficient to specify a three-dimensional location of the SMR. In addition, several trackers are available or have been proposed for measuring six degrees-of-freedom (six-DOF), rather than the ordinary three degrees-of-freedom.
A difficulty that sometimes occurs in using trackers is the losing tracking of a beam of light when an operator passes the retroreflector behind an obstruction or rotates the retroreflector outside its acceptance angle. In these cases, in one mode of operation, the beam of light may remain fixed after tracking is lost. In response, the operator must use other means to recapture the beam. In another mode of operation, the tracker immediately begins seeking the lost retroreflector with cameras affixed to the tracker. However, with this approach, if there are other retroreflectors in the environment, the tracker may redirect the beam of light to the incorrect retroreflector.
An approach sometimes used to distinguish light reflected from retroreflectors from light generated or reflected by other objects in an environment is to provide flashing lights around tracker target cameras and then to subtract alternative camera images. With proper timing of the cameras and lights, this serves to remove constant background lights while retaining the flashing retroreflector lights in camera images. However, flashing lights around the target cameras, when combined with the beam of light from the tracker, may produce undesirable camera patterns when subtracted, causing problems in acquiring retroreflectors.
Although trackers are generally suitable for their intended purpose, the need for improvement remains, particularly in rapidly and flexibly locking onto and tracking retroreflector targets and recovering from the situation in which the tracker beam is sent to the wrong retroreflector.